Evidence for strain control of magnetic anisotropy in epitaxial nickel ferrite thin films grown on strontium titanate substrates
Graphical abstract
Introduction
Spinel ferrites attract extensively attentions with their unique magnetic order parameters, and show promising applications in spintronics, magnetoelectric sensors, and microwave devices [[1], [2], [3], [4], [5], [6]]. The study of magnetic order parameters in thin films of spinel ferrites induced by strain engineering has been of particularly attractive among the variety of controlling approaches. Recently, strain shows potential in inducing a large magnetic anisotropy in spinel ferrites, the understanding and enhancement of magnetic anisotropy has salient impact for the development and utilization of magnetic devices [[7], [8], [9]].
Nickel ferrite (NiFe2O4, NFO), has an inverse spinel structure, with the two ferromagnetic order ions Fe3+ and Ni2+ occupied the tetragonal A sites and octahedral B sites of the spinel structure, respectively, shows high magnetic moment and low damping at room temperature. Recently, the study on magnetic property of NFO thin film in spintronics attracts lots of interests, such as the observation of spin seebeck effect in NFO films attributed to the magnetic anisotropy [10], the tuning of spin hall magnetoresistance in NFO films by interface effect [11], and the angle dependent of spin seebeck effect in NFO films provides a new way to investigate the magnetic anisotropy [12]. Furthermore, the strain control of magnetic anisotropy also have a significant contribution to the magnetic energy as a result of the large magnetostriction of NFO [13]. Several reports have emphasized the importance of the strain effects on the anisotropic properties of pure and substituted NFO thin films [14,15]. Recent reports in this regard include film-substrate lattice mismatch induced anisotropy field Hu as high as 10 kOe for films of NiZnAl-ferrite on (001) MgAl2O4 substrates [16] and Hu ranging from 0.5–11.9 kOe for NFO films on (001) MgAl2O4, MgGa2O4, and CoGa2O4 [17]. Some studies reported a giant saturation magnetization Ms ≈ 771 emu/cm3 for 2.6 nm NFO film on (001) SrTiO3 [18], and the cause of such variations in the magnetization was attributed to the growth mechanisms and antiphase domains boundary in films [6,19]. Most studies of magnetic anisotropy in epitaxial NFO thin film are focused mainly on films with a particular crystallographic orientation, and there is a lack of study aimed at an understanding of the magnetic anisotropy as a function of substrate/film orientation for NFO thin films.
In this work, we report strain control of magnetic anisotropy for epitaxial NFO thin films fabricated on (001)-, (110)-, and (111)-oriented SrTiO3 (STO) single crystal substrates. The choosing of STO substrate is due to the large lattice mismatch of 6.7 % between STO and NFO. We are able to estimate both the magnetization and the growth induced uniaxial anisotropy Hu from the magnetization and FMR measurements. We inferred from these measurements that a large out-of-plane uniaxial anisotropy field Hu is present with the smallest value of Hu =1.4 kOe for NFO/STO (111) and the highest value of 6.1 kOe for NFO/STO (001). The anisotropy fields Hu were compared with estimated values Hσ based on strain in the films and magnetostriction measured on a bulk NFO single crystal. Our results are of importance for the control of the growth induced anisotropy field in thin ferrite films for possible use in spintronic as well as microwave devices.
Section snippets
Experiment
NFO epitaxial thin films were deposited on (001)-, (110)- and (111)-oriented STO substrates by pulsed laser deposition [20]. The fabrication was carried out at 700 °C under an oxygen partial pressure of 15 Pa, the distance between target and substrate is 60 mm. The three oriented NFO thin films were simultaneously deposited on 0.5 mm thick 3 × 3 mm2 STO substrates with a laser energy of 300 mJ at repetition rate of 5 Hz. After the fabrication, all the samples were annealed in air at 800 °C for
Results and discussion
The XRD θ-2θ patterns of NFO thin films grown on (001)-, (110)-, and (111)-oriented STO substrates are shown in Fig. 1. Only the (00 L), (LL0), and (LLL) diffraction peaks of NFO films can be detected, indicating the purity phase and epitaxial growth of NFO films. The epitaxial relationship between NFO thin films and STO substrates can be revealed by RSM, as shown in Fig. 2. The RSMs were collected around the (004), (440), and (444) crystallographic planes for (001)-, (110)- and (111)-oriented
Conclusions
In summary, epitaxial NFO thin films fabricated on (001)-, (110)- and (111)-oriented STO substrates have various strain states due to a large lattice mismatch of 6.7 %. Magnetic and FMR measurements were carried out to determine the magnetization and growth induced out-of-plane uniaxial anisotropy field. The NFO film on STO (111) with highest strain shows the smallest perpendicular uniaxial anisotropy field, and the value of the perpendicular uniaxial anisotropy increases with the decrease of
Author statement
All authors contributed to this work. Y. Liu, Z. Mei, Y. Guo, P. Zhou, C. Dong and A. Adhikary carried out all the experiments. Y. Liu, Z. Mei, P. Zhou, Y. Qi, G. Srinivasan and T. Zhang wrote and revised the manuscript, Y. Qi, K. Liang, Z. Ma, Z. Xia, A. Adhikary, N. Sun, G. Srinivasan, and T. Zhang contributed to the discussion and analysis of the experiments.
Declaration of Competing Interest
The authors report no declarations of interest.
Acknowledgments
This work was financially supported by the National Natural Science Foundation of China (51372074, 11574073, and 11974104), and Hubei Key Laboratory of Ferro & Piezoelectric Materials and Devices (201706). The authors would like to thank Mr. Jingyi Chen of PANalytical Division, Spectris Instrumentation & Systems, Shanghai Ltd. for the assistance with RSM measurements. AA thanks NIH NCI (Grant R01CA045424), Research Excellence Fund (REF), and Center for Biomedical Research at Oakland University
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